Eszopiclone, first disclosed in U.S. Pat. No. 6,444,673, is a short acting nonbenzodiazepine hypnotic agent used in the treatment of insomnia which is the S-isomer of the racemic product zopiclone. As compared to the R-enantiomer, the S-enantiomer is found to be less toxic, binds more specifically to the GABA receptor and shows higher activity. The drug has been marketed in United States by Sepracor under the name Lunesta. Chemically eszopiclone is (S)-(+)-6-(5-chloro-2-pyridinyl)-7-oxo-6,7-dihydro-5H-pyrrolo-[3,4b]-pyrazin-5-yl-4-methyl-piperazine-1-carboxylate (Formula I), and is represented by the following structure.

U.S. Pat. No. 6,444,673 discloses the preparation of eszopiclone which involves resolving racemic zopiclone with O,O′-dibenzoyltartaric acid monohydrate to obtain the corresponding crude salt which is twice recrystallised with acetonitrile to obtain the pure salt (36 mol. % yield). The salt is further alkalinized and recrystallised again with acetonitrile solvent to yield 23 mol. % of eszopiclone.
The process disclosed above involves an excess of solvent in order to obtain pure eszopiclone. Also, the yield of the final product obtained by the process is low.
U.S. Pat. No. 6,339,086 discloses the preparation of eszopiclone using D-malic acid as a resolving agent and a mixture of methanol and acetone as a solvent. For the precipitation of the D-malate salt of eszopiclone, the process requires that the reaction mixture be heated in an oil bath and also seeding with the corresponding salt.
U.S. 2007/0054914 discloses the preparation of eszopiclone which involves resolving zopiclone with di-p-toluoyl-tartaric acid in the presence of an organic solvent. In U.S. 2007/0054914, the resolution step is carried out at room temperature. The problems associated with the process are that under the described conditions the selective crystallization of the desired enantiomer salt does not occur efficiently; hence it is difficult to isolate eszopiclone having a high chiral purity from the reaction mixture. The product has to be further purified at various stages to obtain the desired chiral and HPLC purity.
U.S. 2008/0287447, U.S. 2008/0146800, WO 2007083188, WO 2008126105, and WO 2009063486 disclose the resolution of eszopiclone from racemic zopiclone using various tartaric acid derivatives.
The preparation of eszopiclone described in the prior art processes involves multiple recrystallization in order to obtain an enantiomeric excess of eszopiclone. Further repetitive purifications decrease the yield of product. Also, the multiple recrystallizations cause an increase in the amount of solvent employed, thus making the process environmentally unfriendly and non-economical.
There is, therefore, a need for an improved or alternate process for the synthesis of eszopiclone having a high degree of chiral purity. The present invention is an attempt in providing a simple, economical, eco-friendly, industrially suitable and high yielding process for preparation of eszopiclone.